CV 5

Cardiovascular System Overview

  • Focus on Cardiac Output (CO) and Capillary Exchange

Objectives

  • Understand the dynamics influencing cardiac output.
  • Recognize the forces that affect capillary fluid exchange.

Cardiac Output

  • Definition: The amount of blood the heart pumps in one minute.
  • Equation: CO=HR×SVCO = HR \times SV
    • HR: Heart Rate (beats per minute)
    • SV: Stroke Volume (amount of blood pumped per heartbeat)
Factors Influencing Cardiac Output
  1. Heart Rate (HR)
    • Increase in HR generally raises CO under normal conditions.
    • Exception: Tachycardia can reduce CO due to decreased ventricular filling time.
  2. Stroke Volume (SV)
    • Determined by three main factors:
      • Preload: Volume of blood in the ventricles at the end of diastole.
      • Afterload: Resistance the heart must overcome to eject blood.
      • Contractility: Strength of ventricular contraction.
Changes During Exercise
  • Sympathetic Activation: Increases force of contraction leading to increased SV.
  • Reduction in Peripheral Vascular Resistance: Results in increased blood flow to tissues.
  • Skeletal Muscle Pump: Muscle contractions aid venous return to the heart.

Stroke Volume Regulation

  • SV=EDV−ESVSV = EDV - ESV
    • EDV: End Diastolic Volume (about 120 ml in a healthy heart)
    • ESV: End Systolic Volume (about 50 ml in a healthy heart)
    • Normal SV: About 70 ml, or approximately 60% of EDV.
Components Affecting Stroke Volume
  1. Preload
    • Influenced by:
      • Venous Return: More return increases preload (Frank-Starling mechanism).
  2. Afterload
    • Defined as the stress on the heart wall during ejection (systole).
    • Carbated by resistance, often measured by aortic pressure.
  3. Contractility
    • Independent of preload/afterload; increased by sympathetic stimulation (NE & Epi).

Capillary Fluid Exchange

  • Mechanism: Fluid exchange occurs at the capillary level between blood and interstitial fluid.
  • Key Concepts:
    • Filtration at the arteriole end and reabsorption at the venule end.
    • Maintained by hydrostatic and osmotic pressures.
Forces Influencing Fluid Exchange
  1. Capillary Blood Pressure (BP): Pushes fluid out into interstitial fluid (Filtration).
  2. Interstitial Fluid Hydrostatic Pressure (IFHP): Pushes fluid back into capillaries (Reabsorption).
  3. Blood-colloid Osmotic Pressure (BCOP): Caused by plasma proteins; attracts fluid back into capillaries.
  4. Interstitial Colloid Osmotic Pressure (ICOP): Caused by plasma proteins leaking into interstitial space; encourages fluid outflow into interstitial fluid.
Net Filtration Pressure (NFP)
  • Equation: NFP=(BHP−IFHP)−(BCOP−IFOP)NFP = (BHP - IFHP) - (BCOP - IFOP)
    • Positive NFP: Filtration occurs at arteriole end.
    • Negative NFP: Reabsorption dominates at venule end.

Edema

  • Definition: Accumulation of fluid in the interstitial space.
  • Causes of Edema:
    • Increased capillary permeability (e.g., histamine release).
    • Decreased plasma protein levels (e.g., severe liver damage).
    • Elevated venous pressure due to blockages.
    • Blockage of lymphatic drainage systems.

Sample Questions

  • (1) For a normal heart, what happens when HR increases?
  • (2) What role does the skeletal muscle pump play during physical activity?
  • (3) Define stroke volume and its significance.
  • (4) How does afterload increase affect cardiac output?